#3098 fix: strata_bench defines the port it prints (#3096)

Description

## What it changes

`main` used `port` in two messages but only had the value inline for the address, so both paths crashed with a `NameError`. It now defines `port` once (`--port`, else the config's port, else 8080) and uses it for the address and for both messages.

Two regression tests, one per path that read `port`: the message for a port taken by something else, and the message naming the server the bench starts itself.

## What it leaves alone

No behaviour change. The address, the server start, the exit codes and the report are as they were; the value `port` now holds is the same one that was already used to build `base`.

## How I checked

- Both new tests fail on the unpatched tree with the `NameError` from #3096, at `tools/strata_bench.py:990`.
- `python tools/test_strata_bench.py`: 19 tests, OK. CPU only, no GPU, no model files, Windows, Python 3.14.7.
- The existing `test_no_server_is_an_error_not_a_crash` covers only the `--url` path, which is why this survived.
- Not tested on the reporter's machine (Strix Halo APU, Python 3.15).

Fixes #3096

🤖 Generated with [Claude Code](https://claude.com/claude-code)